Onboard sound or Audigy 2 PCI card?

Onboard sound is probably the worst thing you can use, it's the most basic, stripped down thing you can find, and is pretty much only good for playing error beeps. If you've got the card, install it. If nothing else it'll give you a port in the back to plug in headphones and speakers, plus Soundblaster comes with pretty good software for tweaking the way your sound comes out.